John M. Simonson

The system NaClCaCl2H2O: I. The ice liquidus at 1 atm total pressure

Abstract Phase relations in the ice-stable field of the system NaClCaCl 2 H 2 O have been determined under 1 atm total pressure along the NaClH 2 O and CaCl 2 H 2 O binaries and along five pseudobinaries with constant NaCl /( NaC 1+ CaCl 2 ) weight ratios. The results are in excellent agreement with published data along the NaClH 2 O binary but show large discrepancies when compared to previous determinations of the ice liquidus along the CaCl 2 H 2 O binary and in the NaClCaCl 2 H 2 O ternary. Eutectic temperatures and compositions are −21.20°C (±0.01 °) and 23.2 wt% for the NaClH 2 O binary and −49.95°C (±0.15°) and 30.33 wt% for the CaCl 2 H 2 O binary. The minimum temperature reached during our experiments in the ternary system was −34°C; therefore, we do not report isotherms below −35°C or the ternary eutectic temperature. At moderate to high salinities, isotherms cross the ice sub-field at lower total salt concentrations than previously reported. Fluid inclusion salinities determined from previously published freezing-point data may be in error by as much as 2 wt% total salt. In addition, NaCl /( NaCl + CaCl 2 ) weight ratios estimated from hydrohalite- and ice-melting temperatures may be in error by as much as 0.15.

Thermodynamics of aqueous association and ionization reactions at high temperatures and pressures

Electrochemical and electrical conductance cells have been widely used at ORNL over the years to quantitatively determine equilibrium constants and their salt effects to 300°C (EMF) and 800°C (conductance) at the saturation pressure of water (EMF) and to 4000 bars (conductance). The most precise results to 300°C for a large number of weak acids and bases show very similar thermodynamic behavior, which will be discussed. Results for the ionization constants of water, NH3(aq), HCl(aq), and NaCl(aq), which extend well into the supercritical region, have been fitted in terms of a model with dependence on density and temperature. The entropy change is found to be the driving force for ion-association reactions and this tendency increases (as it must) with increasing temperature at a given pressure. Also, the variation of all thermodynamic properties is greatly reduced at high fixed densities. Considerable variation occurs at low densities. From this analysis, the dependence of the reaction thermodynamics on the P-V-T properties of the solvent is shown, and the implication of large changes in hydration for solutes in the vicinity of the critical temperature will be discussed. Finally, the change in the molar compressibility coefficient for all reactions in water is shown to be the same and dependent only on the compressibility of the solvent.

The enthalpy of dilution of HCl(aq) to 648 K and 40 MPa thermodynamic properties

Abstract Enthalpies of dilution of HCl(aq) have been measured to 648 K and 40 MPa over the molality range of about 0.01 to 15.6 mol·kg −1 . At temperatures above 523 K the experimental results approach infinite dilution with a slope greater than the limiting-law slope, a clear indication of ion association. An analysis of the enthalpies of dilution in combination with existing thermodynamic results has produced a comprehensive model for the thermodynamic properties of HCl(aq). Three versions of the ion-interaction model were used depending on the range of temperature or molality. The unmodified ion-interaction model of Pitzer was adequate through 523 K and 7 mol·kg −1 . Addition of a fourth coefficient extended the molality range to 16 mol·kg −1 through 523 K, and for temperatures above 523 K the second coefficient was given additional dependence on molality (a non-zero β (2) parameter). By using the density of water as an independent variable each version of the ion-interaction model covers the entire range of pressure through 40 MPa and the number of adjustable parameters is minimized. A comparison of the thermodynamic properties of HCl(aq) with those of NaCl(aq) demonstrated that HCl(aq) is somewhat more associated than NaCl(aq) at high temperatures and the enthalpy of ionization makes a significant contribution to the observed apparent relative molar enthalpy of HCl(aq).

Liquid-vapor partitioning of HCl(aq) to 350°C

Partitioning of HCl(aq) between liquid and vapor phases was measured from 50–350°C at 50°C intervals by sampling the liquid and vapor phases in corrosion-resistant autoclaves. Values of the overall partitioning constant K were calculated from the observed compositions of the two phases using published activity coefficients for HCl(aq) in the liquid phase, assuming that the neutral HC1 molecule is the only solute species present in the vapor phase, and assigning a unit fugacity coefficient for vaporphase HC1. Partitioning is a strong function of temperature, with K increasing more than six orders of magnitude over the temperature range of this work. Measured values of K, including results available in the literature to 110°C, are well represented by a simple function of temperature T in Kelvin and solvent density ρ in g · cm−3: logK = −13.4944 − 934.466/T − 11.0029 logρ + 5.4847 logT. The present measurements give directly the composition of the vapor phase over HCl(aq); partitioning of HC1 to the vapor phase over NaCl(aq) brines has been estimated through the use of approximate activity coefficients for HCl(aq) in the mixed brines.

The enthalpy of dilution and apparent molar heat capacity of NaOH(aq) to 523 K and 40 MPa

Abstract The molar enthalpy of dilution Δ dil H m of NaOH(aq) was measured from 6.3 to 0.008 mol · kg −1 at temperatures from 298 to 523 K and pressures from 7 to 40 MPa with a differential flow mixing enthalpy calorimeter. Apparent molar heat capacities C p , φ were measured with a flow heat-capacity calorimeter from 4.0 to 0.1 mol · kg −1 from 323 to 523 K at 7 MPa. These results are correlated with available literature values as functions of temperature, pressure, and molality with the ion-interaction thermodynamic treatment. The extended form of the ion-interaction model, including an approximation for the effect of ion association, was needed to represent the enthalpy results quantitatively above 473.15 K; this additional term does not have a large effect on the activity or osmotic coefficients. Standard heat capacities calculated from this work are consistent with literature values for the heat-capacity change on ionization of water.

CaCl2(aq) at elevated temperatures. Enthalpies of dilution, isopiestic molalities, and thermodynamic properties

Enthalpies of dilution of CaCI2(aq) have been measured for molalities ranging from 0.008 mol·kg−1 to 7.26 mol · kg−1, at temperatures from about 298 K to 526 K, and for pressures of approximately 7 MPa to 40 M Pa. Isopiestic molalities of CaCI2(aq) were measured over the temperature range of 444 K to 524 K at molalities ranging from 0.55 mol · kg−1 to 4.8 mol · kg−1 with NaCl(aq) serving as the reference electrolyte. The results can be described quite well within the framework of the ion-interaction model. Our results have been combined with other thermodynamic results for CaCl2(aq), including heat capacities and densities, to produce a general model for the thermodynamic properties of CaCl2(aq) which is valid for molalities to 4.6 mol · kg−1 and pressures to 40 MPa over the temperature range 270 K to 526 K. The model is based on the ion-interaction treatment of Pitzer and allows for a small amount of ion association at the highest temperatures by using the β(2) parameter in the second virial coefficient and making the α1 parameter linearly dependent on temperature. For temperatures greater than 523 K, a description of the thermodynamic properties of CaCl2(aq) will require explicit consideration of ion association.

Thermodynamic properties of Aqueous Magnesium Chloride Solutions from 250 to 600 K and to 100 MPa

A new general model that describes the thermodynamic properties of MgCl2(aq) has been developed from a global fit to experimental results, including isopiestic molalities, vapor pressure measurements, freezing-point depressions, enthalpies of dilution, heat capacities, and densities, for this system. The model is based on a recent ion-interaction treatment with extended higher-order virial terms, and on experimental results from 240 to 627 K at pressures to 100 MPa and molalities to 25 mol⋅kg−1.

Excess molar enthalpies and the thermodynamics of (methanol + water) to 573 K and 40 MPa

Abstract Excess molar enthalpies H m E of (methanol + water) were measured from 298 to 573 K at pressures from 7 to 40 MPa. The observed H m E s are significantly negative at 298.15 K, near zero at about 398 K, and positive at higher temperatures. The composition dependence of H m E at low temperatures is much simpler than for (ethanol or propanol + water). The measurements reported here extend above the critical temperature of methanol. The very large changes in H m E with temperature and composition often observed near the critical conditions of the more volatile component were not observed under the conditions of this study. The results reported here were combined with available measured values of other thermodynamic quantities and represented with thermodynamically consistent empirical functions of temperature, pressure, and composition. Calculated excess molar Gibbs free energies, excess molar volumes, excess molar enthalpies, and excess molar heat capacities are tabulated.

Apparent molar volumes of aqueous calcium chloride to 250°C, 400 bars, and from molalities of 0.242 to 6.150

Relative densities of CaCl2(aq) with 0.22≤ml(mol-kg−1)≤6.150 were measured with vibrating- tube densimeters between 25 and 250°C and near 70 and 400 bars. Apparent molar volumes VΦ calculated from the measured density differences were represented with the Pitzer ioninteraction treatment, with appropriate expressions chosen for the temperature and pressure dependence of the virial coefficients of the model. It was found that the partial molar volume at infinite dilution VΦo, and the second and third virial coefficients BV and CV, were necessary to represent VΦ near the estimated experimental uncertainty. The ionic-strength dependent β(1)v term in the BV coefficient was included in the fit. The representation for VΦ has been integrated with respect to pressure to establish the pressure dependence of excess free energies over the temperature range studied. The volumetric data indicate that the logarithm of the mean ionic activity coefficient, ln γ±(CaCl2), increases by a maximum of 0.3 at 400 bars, 250°C, and 6 mol-kg−1 as compared with its value at saturation pressure.

The Thermodynamics of Aqueous Borate Solutions. II. Mixtures of Boric Acid with Calcium or Magnesium Borate and Chloride

AbstractPotentials for the cell without liquid junction